WITH A LIST OF SPECIES OBSERVED. 227 



mistaken for it. Professor E. von Marten has also noticed this, 

 although he believes that it differs ixom jenkinsi in other respects. 



It seems extremely probable that this new species has been 

 introduced from abroad, particularly as it has not been noticed in 

 any other part of Great Britain. From the fact that none of our 

 other Hydrobias seem to have the least tendency to assume the 

 keeled and tufted appearance, surely, if indigenous, it must have 

 been noticed long ago. It seems hardly possible that Dr. Jeffreys, 

 Mr. Marshall, and other conchologists of repute, could have over- 

 looked the species. But, supposing it to be a recent introduction, we 

 are met with the further difficulty that it is almost inconceivable that 

 the Hydrobia could have increased to such an extent in a few years. 

 The shells now occupy many miles of ditches upon both sides of the 

 Thames, and from their greater vigour and prolificness they seem 

 likely before long to predominate over our other native species. 



The carinated tufted specimens oi H.jenkitisi "axq so distinct, and 

 generally they so far exceed the proportion of shells which are not 

 keeled or tufted, that I prefer to consider them as typical of the 

 species ; those specimens which are without any trace of either keel 

 or tufts may be called var. ecarinata. Besides these two forms, there 

 are to be found existing with the type at Beckton, and several other 

 places, a very short spired, much inflated variety, which I have pro- 

 posed to name var. tnmida. Other specimens occur at Beckton and 

 Plumstead which are much more graceful in contour, the spire is long 

 and tapering, and the outer lip very much expanded ; if worthy of 

 being considered as a variety, they might be designated var. gracilis. 



There is every reason to believe that the pretty little shell H. 

 similis (or H. confusa, Frau., as they prefer to call it at the Natural 

 History Museum at Kensington), will not long continue to be 

 enumerated with the British Hydrobiae. A recent visit to its habitat 

 resulted in obtaining two dead shells only, and the most diligent 

 search for living specimens was unavailing. It is just possible that a 

 few may still remain in hybernation, as the locality has been known to 

 but a few conchologists, who have done their best to preserve it from 

 extinction. 



In searching for this Mollusc some very interesting caddis-worm 

 (Phryganidifi) cases have at times been collected, which have been 

 constructed almost entirely of the young shells of H. si/iiilis and of 

 H. ventrosa ; and upon one occasion I came across a caddis-case 

 which had a single shell of the subterrannean species Cixcilioides 



